Systems and Methods for Decay Heat Removal from an Exterior of a Nuclear Reactor

1 FIG. 1 FIG. 8 2 1 1 12 3 1 2 1 2 16 8 16 18 2 1 8 is a profile cross-section of a related art liquid metal nuclear reactor, such as that described in co-owned U.S. Pat. No. 5,406,602 to Hunsbedt et al. issued Apr. 11, 1995, incorporated herein in its entirety by reference. As seen in, annular or circular concrete silo, potentially underground, houses annular guard vesselthat in turn houses reactor, potentially all concentrically aligned. Reactorincludes a nuclear reactor coresubmerged in a liquid metal coolant, such as liquid sodium. A space, shown as gap, between reactorand guard vesselmay be filled with an inert gas, such as argon. Reactorand guard vesselare suspended vertically downward from upper frame. Concrete silomay support upper frameby seismic isolatorsto maintain structural integrity of guard vesseland reactorduring earthquakes and allow uncoupled movement between those structures and surrounding silo.

1 15 12 1 15 12 12 1 1 2 2 8 Reactoris controlled by neutron-absorbing control rodsselectively inserted into or withdrawn from reactor core. Reactormay be shut down entirely for responding to an emergency condition or performing routine maintenance by inserting control rodsinto coreof fissionable fuel to deprive the fuel of the needed fission-producing neutrons. However, residual decay heat continues to be generated from coredecreasing exponentially over time. This heat must be dissipated from shut-down reactor. The heat capacity of the liquid metal coolant and adjacent reactor structures aid in dissipating the residual heat. For instance, heat may be transferred by thermal radiation from reactorto guard vessel. Heat from guard vesselmay also radiate outwardly toward concrete silospaced outwardly therefrom.

1 5 2 8 4 2 5 5 7 8 5 2 4 5 2 9 4 6 6 7 8 4 9 1 FIG. Systems for removal of this decay heat vent or otherwise remove the heat from reactorand surround structures to a heat sink such as the environment. One such system may be a reactor vessel auxiliary cooling system (RVACS) as shown in. Heat collector cylindermay be concentrically between guard vesseland siloand define hot air riserbetween guard vesseland an inner surface of heat collector cylinder. Heat collector cylindermay further define cold air downcomerbetween siloand an outer surface of heat collector cylinder. Heat may be transferred from guard vesselto air in hot air riser. The inner surface of heat collector cylindermay receive thermal radiation from guard vessel, with the heat therefrom being transferred by natural convection into the rising air for upward flow to remove the heat via air outlets. Heating of the air in riserby the two surrounding hot surfaces induces natural air draft in the system with atmospheric air entering through air inletsabove ground level. The air from inletsis ducted to cold air downcomer, then to the bottom of concrete silo, where it turns and enters hot air riser. The hot air is ducted to air outletsabove ground level. Similar, related passive reactor coolant systems are described in U.S. Pat. No. 5,190,720 to Hunsbedt et al., issued Mar. 2, 1993, and U.S. Pat. No. 8,873,697 to Horie et al., issued Oct. 28, 2014, all of which are incorporated herein by reference in their entireties.

This background provides a useful baseline or starting point from which to better understand some example embodiments discussed below. Except for any clearly-identified third-party subject matter, likely separately submitted, this Background and any figures are by the Inventor(s), created for purposes of this application. Nothing in this application is necessarily known or represented as prior art.

Example embodiments include systems to flood fillable areas around nuclear reactors to sink heat from the same. A containment structure or building, such as a silo underground or rebar concrete dome, can separate the reactor nearly completely from surrounding environment while also containing open space around and/or under the reactor. Example systems include at least one reservoir of coolant connected to the containment that can be opened and/or closed to flood the coolant into the containment up to the reactor. Sizing of the reservoirs, containments, and/or flow paths therebetween may ensure coolant fills up to touch the reactor but does not block other needed flow paths around the reactor, such as an RVACS air flow space. The reactor may include insulation, such as a guard layer or air gap, on the pressure vessel of the reactor to reduce damage or other operational interference caused by extreme temperature differences and heat capacities between the coolant and pressure vessel. For example, using liquid water from a spent fuel pool or in-containment tank to flood the area immediately surrounding the reactor may be negotiated via an air-gapped guard vessel on the pressure vessel to ensure no overly-rapid quenching, steam explosion, temperature expansion/constriction gradient, etc. occurs to the reactor.

Example systems may be installed and operated at any point in plant life. This may allow other safety systems, potentially already existing, to be operated not as last-resort or safety-regulated systems. For example, an RVACS may be outfitted with electrically-powered blowers or cut-offs for improved performance, or surrounding rock pile for added security, without risk of performance degradation in the RVACS ultimately threatening loss of decay heat sinking. Methods of installation may provide a coolant source and connect the same to the containment, such as an external tank of natural body of water, through a pipe or line running underground, or with an internal tank like a holding annulus inside a perimeter of the containment structure that floods the same. The sources may be supplied or filled with volumes of coolant sufficient to fill spaces around the reactor with heat-sinking coolant, while not over-flooding or blocking other cooling components.

Because this is a patent document, general broad rules of construction should be applied when reading it. Everything described and shown in this document is an example of subject matter falling within the scope of the claims, appended below. Any specific structural and functional details disclosed herein are merely for purposes of describing how to make and use examples. Several different embodiments and methods not specifically disclosed herein may fall within the claim scope; as such, the claims may be embodied in many alternate forms and should not be construed as limited to only examples set forth herein.

Membership terms like “comprises,” “includes,” “has,” or “with” reflect the presence of stated features, characteristics, steps, operations, elements, and/or components, but do not themselves preclude the presence or addition of one or more other features, characteristics, steps, operations, elements, components, and/or groups thereof. Rather, exclusive modifiers like “only” or “singular” may preclude presence or addition of other subject matter in modified terms. The use of permissive terms like “may” or “can” reflect optionality such that modified terms are not necessarily present, but absence of permissive terms does not reflect compulsion. In listing items in example embodiments, conjunctions and inclusive terms like “and,” “with,” and “or” include all combinations of one or more of the listed items without exclusion of non-listed items. The use of “etc.” is defined as “et cetera” and indicates the inclusion of all other elements belonging to the same group of the preceding items, in any “and/or” combination(s). Modifiers “first,” “second,” “another,” etc. do not confine modified items to any order. These terms are used only to distinguish one element from another; where there are “second” or higher ordinals, there merely must be that many number of elements, without necessarily any difference or other relationship among those elements.

When an element is related, such as by being “connected,” “coupled,” “on,” “attached,” “fixed,” etc., to another element, it can be directly connected to the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly connected,” “directly coupled,” etc. to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.).

As used herein, singular forms like “a,” “an,” and “the” are intended to include both the singular and plural forms, unless the language explicitly indicates otherwise. Indefinite articles like “a” and “an” introduce or refer to any modified term, both previously-introduced and not, while definite articles like “the” refer to the same previously-introduced term. Relative terms such as “almost” or “more” and terms of degree such as “approximately” or “substantially” reflect 10% variance in modified values or, where understood by the skilled artisan in the technological context, the full range of imprecision that still achieves functionality of modified terms. Precision and non-variance are expressed by contrary terms like “exactly.”

The structures and operations discussed below may occur out of the order described and/or noted in the figures. For example, two operations and/or figures shown in succession may be executed concurrently or may be executed in the reverse order, depending upon the functionality/acts involved. Similarly, individual operations within example methods described below may be executed repetitively, individually or sequentially, so as to provide looping or other series of operations aside from exact operations described below. It should be presumed that any embodiment or method having features and functionality described below, in any workable combination, falls within the scope of example embodiments.

Proportions, sizes, and shapes shown in the figures are examples for illustration. While they reflect features of some example embodiments, other relationships and magnitudes of dimensions are included in these examples. As used herein, “angular” directions substantially follow a rounded perimeter of a referenced feature, and “radial” directions substantially follow a radius of that rounded perimeter, perpendicular to the angular direction. “Vertical” and height directions substantially follow an up-down orientation, orthogonal to the radial and angular directions of a referenced feature and more likely oriented with gravity.

The inventors have recognized that existing containment buildings and reactor housing structures are conventionally kept isolated and dry, with no free-standing coolant therein that might contact the reactor. Direct contact with a reactor or nearby structure with a higher heat-sinking coolant, such as liquid water, may represent a risk of too great a temperature change or variation within a pressure vessel like those forming many types of reactors. Other safety systems are instead used to inject coolant directly into the reactor, and/or provide gaseous or air ventilation about the reactor, such as with an RVACS with lower heat-sinking coolant. Because these may be safety-related systems to dissipate decay heat, they may need to be completely passive and unencumbered, such that powered fans or dampers, or blocking shields that might protect the same from external threats, cannot be used where they may interfere with such heat-sinking in transient scenarios. For an RVACS, this further counsels for a dry containment with no other liquid coolant around a reactor that might block airflow around the same. The inventors have thus recognized multiple problems in otherwise sinking decay heat from a reactor while simultaneously improving safety and operations in existing coolant systems, especially in introducing a further coolant into containment that might directly sink heat from a reactor exterior. To overcome these newly-recognized problems as well as others, the inventors have developed example embodiments and methods described below to address these and other problems recognized by the inventors with unique solutions enabled by example embodiments.

The present invention is systems for introducing a coolant around a nuclear reactor exterior, and methods of installing and operating the same as well as relieving and allowing reconfiguration of additional safety systems with the same. In contrast to the present invention, the few example embodiments and example methods discussed below illustrate just a subset of the variety of different configurations that can be used as and/or in connection with the present invention.

2 FIG. 1 FIG. 100 8 1 100 101 101 102 102 8 13 1 101 102 8 1 is a schematic illustration of an example embodiment flooding systemuseable to cool a nuclear reactor inside a containment structure, such as a nuclear reactor silohousing reactorand a passive coolant system such as an RVACS of. Example embodiment systemincludes coolant sourcehousing a large amount of heat sink coolant, such as liquid water. Coolant sourceis connected by flow pathto provide the coolant to directly absorb heat from the reactor. For example, flow pathmay open into an open, fillable area in siloabout bottomof reactor. Coolant may flow from source, through flow pathinto siloand directly contact an outside of reactor, sinking heat from the same without injection into fuel or other reactor internals.

101 8 101 101 8 102 2 FIG. Coolant sourcemay be an external tank as shown inhousing thousands of gallons of liquid water or more. Such an external tank may be outside siloand/or any other containment structure, to allow external filing, purification, refilling, etc. within involvement with containment. For example, sourcemay be an internal or external coolant pool, natural body of water, spent fuel pool, etc. Sourcemay further be several sources, such as a nearby river, onsite holding tank, and spent fuel pool all connected to flow into siloupon opening of flow path.

3 FIG. 101 201 8 201 1 201 13 102 201 Or, for example, as shown in, sourcemay be annular tankentirely within a containment such as silo. Annular tankmay extend about a partial or entire outer perimeter of the containment, from a floor or other height to a substantial elevation around reactor. Annular tankmay flow directly into an area below reactor, with a minimal or nonexistent flow path, such as valving under operator manual control or plant system automated actuation, a rupture disk, and/or a frangible seam in tankconfigured to fail open at threshold temperatures or transient conditions.

2 FIG. 10 8 10 101 102 8 10 8 further illustrates rock pile, which optionally provides security and access restriction to RVACS entrances about siloor other reactor containment structure. A similar configuration is described in U.S. Pat. No. 11,443,859 to Bass et al., incorporated herein by reference in its entirety. Rock pilemay similarly extend around sourceand flow path. Similarly, coolant provided to silomay evaporate or flow out of RVACS, a relief line, or another vent, out of rock pilewithout becoming blocked or building pressuring in silo.

102 13 1 101 8 103 13 102 8 102 8 101 8 101 102 1 Flow pathmay open about bottomof reactor, allowing coolant from sourceto flow into and flood siloup to impingement levelwhere the coolant contacts reactor bottom. For example, flow pathmay be piping or another channel running underground to a bottom of silobelow ground. Pumps and/or valves on flow pathmay control a flow of liquid coolant into silo. Additionally or alternatively, coolant may flow from sourceinto silothrough gravity alone, such as following opening of a valve or rupture disc at a transient threshold or operator action. In this way, coolant sourceand flow pathmay passively provide coolant to contact reactorwithout the need for outside power or operator action or monitoring.

103 4 7 5 104 101 4 7 5 Impingement levelof the coolant liquid may be below a lowest point of a divider between hot air riserand cold air downcomerin heat collector cylinder. This may preserve an above-liquid passagefor air or other coolant less dense than that used in sourceto pass between riserand downcomer. In this way, coolant flow, such as airflow, through cylinderand an RVACS in general may still be possible in example embodiments, and even the use of a heavy liquid coolant may not block flow through an RVACS.

103 104 101 8 104 101 101 104 8 8 13 1 102 103 102 103 Impingement levelmay be ensured at levels to preserve passagein any manner, including limiting total volume of sourceto that less than a volume of liquid within silobelow passage, by positioning sourcesuch that the coolant level in sourceis at passageand preserves the same when flowed into silo, by limiting flow rates into siloto those expected to match evaporation, vaporization, and/or outflow of the coolant once in contact with bottomof reactor, such as through valves, pump speeds, dimensions of flow path, etc., overflow drains open only at or above level, and/or via cut-off triggers that close flow pathwhen coolant level reaches impingement level, etc. The volume and levels of coolant necessary to both reach the reactor and not interfere with other in-containment coolant structures can be readily determined from the amount of free volume inside a containment around a reactor, with maintenance and make-up matching expected evaporation, vaporization, leakage, etc. rates.

13 1 101 1 101 Contact at lower surface of reactorby the coolant may be direct or indirect, such as through a guard vessel. If the coolant contacts sufficient surface area, even at elevated temperatures a modest amount of coolant may be sufficient to remove decay heat when reactoris scrammed or shut down. For example, 6 gallons per minute of water provided from sourceand evaporated by decay heat from reactormay represent a thermal megawatt of heat sinking provided to the reactor. Needing to sink decay heat, which may be less than 7% of a reactor's rated thermal power at shutdown and exponentially reducing to 1.5% of the same after an hour, may thus require a relatively small amount of water provided, such that a spent fuel pool or other flexible source may be used for source.

100 110 4 7 100 110 100 100 Because the flooding and coolant contact provided in example embodiment systemmay provide relatively large amounts of passive decay heat sinking, potentially for days or weeks post-shutdown, the RVACS and any other reactor safety system may not need to be relied on for safety purposes. For example, powered ventilation systemin RVACS riseror downcomerthat actively drive airflow may not be useable or relied upon as safety systems because of their active power requirement. With example embodiment systemin place, fans and/or dampers in ventilation systemmay be used to enhance air flow through the RVACS in conjunction with systemor at any other desired time. Similarly, RVACS could be passively sealed or otherwise taken out of service to enhance plant thermodynamics because systemmay be relied upon to provide safety cooling. For example, damper systems for closing an RVACS disclosed in U.S. Pat. No. 10,937,557 to Sineath et al, incorporated by reference herein in its entirety, may be used without any risk of loss of RVACS function when needed as a safety system of last resort.

4 FIG. 302 1 1 13 1 302 13 1 302 302 1 302 8 302 100 302 1 1 is a detail illustration of insulated guard vesselof reactorconfigured to directly contact a higher heat-sinking coolant in example systems while protecting the pressure vessel of reactorfrom too rapid of a quench. For example, using 15-20° C. water as the flood coolant about bottomof reactorat approximately 190° C., insulated guard vesselmay prevent the pressure vessel walls at bottomof reactorfrom cooling more than approximately 0.2-1° C. per second, such as a 0.67° C. per second limit on cooling rate of the vessel wall under these operating conditions. An intermediate boundary provided by guard vesselmay reduce surface temperature differential thus eliminating any material concerns from too rapid of a quench or extreme temperature gradient in the pressure vessel wall or from steam explosions or thin-film insulation formation from overheating the coolant. Such insulation may be provided through any material, including an inert air gap between an outer pressure vessel surface and guard vessel. The pressure vessel of reactormay be completely surrounded by guard vesselup to a support flange at a top of silo, with securing contacts between the vessels and at penetrations, or guard vesselmay be only at selective positionings, such as only those elevations or locations expected to be directly impinged by the coolant in example systems. In this way, a flooding coolant in example systemmay directly contact guard vessel, remove sufficient decay heat to fully cool shutdown reactor, and boil off or flow through the RVACS or other relief line, without risk of too rapidly quenching the pressure vessel of reactoror needing any other safety system.

100 100 8 101 102 101 13 1 302 1 Example embodiment flooding systemcan be installed at plant fabrication or at any point at plant life. For example, systemmay be installed in an existing containment such as silo, potentially with an RVACS system, or added to the same during plant construction by installing sourcewithin or near the containment, potentially with flow path. For example, an existing spent fuel pool as sourcemay be connected to an opening in containment with proper controls and valving to allow water in the pool to reach elevations of at least bottomof reactor. Similarly, insulated guard vesselmay be installed on a bare reactorat fabrication or installation, or may replace an existing guard vessel or be fabricated from an existing guard by adding sufficient insulation to allow direct coolant contact without damage to the reactor.

100 101 1 1 100 100 Once installed, example embodiment systemmay be activated by allowing coolant to flow from sourceinto the containment and up to an elevation of reactor. This may be achieved passively by simply opening a flow path like a valve and allowing the coolant to drain into the containment under gravity. As coolant directly contacts a pressure vessel and/or guard vessel of reactor, it sinks heat from the same, potentially all decay heat from the same. The evaporated, vaporized, or boiled coolant may then exit, such as through an RVACS or other relief line. This flooding may be performed during a transient event, at installation, during a testing event, such as during a maintenance outage, and/or at any other time of desired cooling. While example embodiment systemis installed, other systems may be used in a non-safety and non-passive manner. For example, active fans, damper structures, and other uses of an RVACS to enhance its operation may be used when systemis installed.

100 Example embodiment systemmay be fabricated of resilient materials that are compatible with a nuclear reactor environment without substantially changing in physical properties, such as becoming substantially radioactive, melting, embrittlement, and/or retaining/adsorbing radioactive particulates. For example, several known structural materials, including austenitic stainless steels 304 or 316, XM-19, zirconium alloys, nickel alloys, Alloy 600, etc. may be chosen for any element of components of example embodiment systems, including annular tanks and guard vessel in closer proximity to a reactor. Joining structures and directly-touching elements may be chosen of different and compatible materials to prevent fouling.

Example embodiments and methods thus being described, it will be appreciated by one skilled in the art that example embodiments may be varied and substituted through routine experimentation while still falling within the scope of the following claims. For example, while some example embodiments are shown in use in combination with reactors having an RVACS, other reactor containments and plants can be used simply through proper positioning and sizing of example embodiments—and fall within the scope of the claims. Such variations are not to be regarded as departure from the scope of these claims.